1. Field of the Invention
The field of the invention is that of synchronizing an analog modem receiving signals such as radio signals to receive digital data packets.
2. Description of the Prior Art
To be more precise, the present invention concerns a device adapted to detect what is referred to as a singular word in the preambles of TDMA (time-division multiple access) packets. The device of the invention can, among other things, be used for the transmission of signals by satellite. The invention also concerns a detection method used in a device of this kind.
FIG. 1 shows the structure of a TDMA packet 10. The packet 10 includes a preamble comprising words RP, RR and MU. This preamble is followed by data D constituting the wanted information.
The word RP is a carrier recovery preamble and comprises a succession of identical symbols, for example the symbol 11 (unmodulated carrier).
The word RR is a timing recovery preamble and comprises alternating symbols, for example symbols 00 alternating with symbols 11 or symbols 10 alternating with symbols 01.
The word MU is the singular word and is used to identify the first symbol of the data D. For example, it comprises 32 symbols taking their value from the set {00, 11} (QPSK modulation where only two opposite states are used). Hereinafter the symbol 00 corresponds to a 0 sample and the symbol 11 corresponds to a 1 sample. The position of this singular word MU is detected by correlation at a modem receiving such packets.
In the transmission system to which the present invention relates the data is QPSK modulated (Quaternary Phase Shift Keying) and the packet preambles use only two opposite states of the constellation of four possible states so that this modulation can be regarded as BPSK modulation (Binary Phase Shift Keying).
The packets received by a modem usually originate from several remote stations and it is necessary to synchronize the various stations so that two different stations do not transmit simultaneously in the same time period. However, synchronizing the stations does not prevent time and frequency drift affecting the packets received by the modem, caused in particular by the motion of the satellite and by the fact that the transmission medium may be heterogeneous. Accordingly, each packet is received at a random time, within a time window, and on a random frequency, within a frequency window. The modem must therefore be capable of synchronizing in time and in frequency to each new packet received during a call.
This disclosure is concerned only with time synchronization, which proceeds in two phases:
timing synchronization, which consists in determining the time at which the eye diagram of the received signal is at its most open, in order to optimize the signal sampling time; timing synchronization uses the timing recovery word RR; PA1 packet synchronization, which consists in determining the time of arrival of each packet and thus the position of the first symbol of the wanted information D. PA1 means applying an angular offset of .pi./4 to the constellation of said complex envelope, said angular offset means supplying a complex signal on a main channel and on an auxiliary channel each comprising a real channel and an imaginary channel; PA1 N sample delay means on said main channel supplying time-delayed samples to a multiplexer on a time-delayed real channel and a time-delayed imaginary channel; PA1 estimator means on said auxiliary channel to estimate which of said real and imaginary channels of said main channel is conveying the greater energy, said estimator means supplying information to said multiplexer so that the latter supplies at its output whichever of the time-delayed real or imaginary channels of said main channel corresponds to that of said channels of said auxiliary channel conveying the greater energy; PA1 a correlator receiving a reference singular word and computing a correlation level between the output signal of said multiplexer and said reference singular word; PA1 comparator means comparing said correlation level with a threshold value to indicate detection of said singular word in the signal of said channel applied to said correlator if said correlation level is greater than said threshold value. PA1 means comparing said summed energies, said comparator means supplying said information to said multiplexer so that the latter supplies at its output whichever of said time-delayed real or imaginary channels corresponds to that of said channels of said auxiliary channel for which the sum of the energies is the greater. PA1 means computing the detected energy difference between each sample of said real channel and each sample of said imaginary channel received at the same time; PA1 means summing said energy differences over N samples; PA1 means comparing the sum of said energy differences with a reference value substantially equal to half the sum of said energy differences, said comparator means generating said information supplied to said multiplexer. PA1 shifting by .pi./4 the constellation of the complex envelope of a signal output by a carrier recovery device; PA1 detecting which real or imaginary channel of said constellation carries the greater energy and computing a correlation level between the digital signal conveyed by this channel and a reference singular word; PA1 supplying an indication of detection of said singular word in said signal of said channel conveying the greater energy if said correlation level exceeds a predetermined value.
The present invention is more particularly concerned with packet synchronization.
FIG. 2 shows a device 28 for recovering the carrier of a signal SR received by a TDMA analog modem.
The signal SR is applied to two mixers 20 and 21 and to a circuit 22 which raises the signal SR to the power 4 in order to eliminate the phase modulation. The circuit 22 is followed by a bandpass filter 23 centered on the frequency 4Fo where Fo is the carrier frequency of the channel concerned. The filter 23 drives a frequency divider 24 which divides by 4 and supplies the recovered carrier to the mixer 20 and by a phases-shifter 25 which applies a phase-shift of .pi./2 and drives the mixer 21. The output signals of the mixers 20 and 21 are applied to sampling devices 26 and 27 clocked by a clock signal at the symbol frequency Hs. The output signals of the sampling devices 26 and 27 constitute two channels P and Q conveying samples in phase quadrature.
The P and Q channel samples are fed to a demodulator and to a singular word detector device shown in FIG. 3. The function of this device is to indicate that a singular word has been detected in the received signal, which enables it to be located in time, and thus to indicate that the wanted information samples are available.
The P and Q channel samples are fed to level comparators 31, 32 which output a regenerated sample for each sample received. The regenerated sample has the value 1 if the received sample is positive or 0 if the received sample is negative, for example. The comparators are followed by identical correlators 33, 34 each receiving the singular word MU constituting the reference word.
FIG. 4 shows one of the correlators. The samples from the comparator on its input side are fed to an input E and pass through a succession of flip-flops 40 clocked at the symbol frequency Fs. By means of exclusive-OR gates 41, they are successively compared with the bits C.sub.0 through C.sub.N-1 of the reference singular word MU. The number of flip-flops 40 is equal to the number of bits N in the singular word MU. The outputs of the gates 41 are summed by adders 42 which supply a correlation level Ns.
The correlator computes in each symbol time the Hamming distance between the samples received and the singular word. The correlation level Ns is equal to 0 if the correlation is maximal, i.e. if the bits supplied to the inputs of each gate 41 are identical, whereas it is maximal (approximately equal to N/2) if correlation is totally absent. In existing modems the singular word MU has the lowest possible correlation with the timing recovery word RR which precedes it so that the correlation search window can be opened before the word MU appears.
Referring to FIG. 3, the respective output levels Np and Nq of the correlators 33 and 34 are fed to combining means 35. The combining means 35 supply an output signal of level s equal to: EQU s=MAX (.vertline.N--Np--Nq.vertline., .vertline.Np--Nq.vertline.)
where MAX is the function selecting the greater of the values .vertline.N--Np--Nq.vertline. and .vertline.Np--Nq.vertline.. Thus if correlation is detected, s=N (Np=Nq=0), whereas in the absence of correlation, s.apprxeq.0 (Np=Nq=N/2).
The output level s of the combining means 35 is fed to a subtractor 36 receiving a threshold value S. This subtractor supplies the value s-S to a decision unit 37 indicating whether correlation has been detected, i.e. if the singular word MU has been detected.
The threshold value S is chosen according to the noise level on the transmission link, as a compromise between a low value to enable detection of the singular word and a high value so that the decision unit 37 does not generate false alarms (spurious singular word detection).
The problem that arises with a singular word detector device of this type is that before the introduction of error correcting codes for TDMA transmission modems operated with a high signal to noise ratio. The operating threshold was around a signal to noise ratio of 9 dB for the transmission channel in question, for a bit error rate of less than 10.sup.-4. This made it possible to use a carrier recovery filter with a relatively wide bandwidth, enabling rapid acquisition of the carrier frequency.
The introduction of dedicated decoder circuits operating at high speeds now makes it possible to use high-performance error correcting codes. The use of these techniques has reduced the operating threshold considerably: it is now possible to achieve a transmission bit error rate of below 10.sup.-4 with a signal to noise ratio of only 3 dB after coding the information to be transmitted. This performance can be achieved with a 1/2 rate convolutional code and a constraint length of 7, for example.
The bandwidth of the phase recovery filter 23 in FIG. 2 has to be set according to the signal to noise ratio. If the ratio is high, it is possible to use a relatively wide bandwidth, producing a short acquisition time. Variation in the carrier frequency can then be dealt with quickly. On the other hand, if the ratio is low, as in the case where error correcting codes are used, the bandwidth must be reduced to minimize the probability of cycle skipping due to the divider 24 in FIG. 2. This reduction in the bandwidth is to the detriment of the acquisition time and there is therefore the risk that the carrier recovery loop will not have stabilized by the end of the preamble, which produces a bit error rate at the start of the packet, and in particular on the singular word, greater than that achieved in the middle and at the end of the packet.
This phenomenon is illustrated by curves 11 and 12 in FIG. 5 showing the phase .phi. of the modem carrier for a narrow loop bandwidth (curve 11) and a wide loop bandwidth (curve 12), for two successive packets 13, 14 which are also shown.
The modem receives a packet 13 including data D.sub.13. This packet is followed by another packet 14 including a preamble as previously described and data D.sub.14. When the packet 13 is decoded the carrier frequency generator of the modem has a phase .phi..sub.13. At the end of the packet 13 the modem receives noise and the phase of the carrier varies erratically until the start of the preamble of packet 14. The phase of the packet 14 carrier is .phi..sub.14 and the modem frequency generator generates a frequency whose phase is .phi..sub.14.
For a narrow loop bandwidth there remains a phase error .DELTA..phi. at the start of the wanted information D.sub.14 although a wide loop bandwidth would have enabled fast stabilization of the phase well before the appearance of the wanted information. However, it is not possible to use a wide loop bandwidth because of cycle skipping (rotation of the constellation of the received signal).
The reduction in signal to noise ratio due to the use of error correcting codes has obliged manufacturers to increase the length of the preamble (words RP and RR) of each packet to ensure that at the end of each preamble there is no carrier phase error. The solution entailing lengthening the preamble is not acceptable as the efficiency of the packets (number of synchronization samples/total number of samples in a packet) is then reduced.
An object of the present invention is to provide a device for detecting a BPSK modulated singular word without requiring longer preambles than the prior art.
Another object of the invention is to enable rapid stabilization of the detected carrier phase, i.e. the use of a wide carrier recovery loop bandwidth, without the cycle skipping that can otherwise affect the received signal. A further object of the invention is to provide a method of detecting the singular word in a TDMA signal frame.